The present invention relates to semiconductor laser devices, and in particular, to a semiconductor laser device capable of achieving especially high power, high reliability, and long operating life and an optical disk reproducing and recording apparatus that employs the device.
Semiconductor laser devices are used for optical communication apparatuses, optical recording apparatuses and so on. In recent years, there have been increasing needs for making the devices have high operating speed and large capacities, and there is promoted research and development for improving various characteristics of the semiconductor laser devices to respond to the needs.
Among them, a 780-nm band semiconductor laser device to be used for an optical disk reproducing (recording) apparatus of CD, CD-R/RW and the like has conventionally been made of an AlGaAs-based material. There have also been growing demands for high-speed write on CD-R/RW, and the semiconductor laser device has been required to yield a high output in order to cope with the demands.
FIG. 11 shows a schematic view of a conventional AlGaAs-based semiconductor laser device. In this semiconductor laser device, an n-GaAs buffer layer 502, an n-Al0.458Ga0.542As first lower clad layer 503, an n-Al0.5Ga0.5As second lower clad layer 504, an Al0.328Ga0.672As lower guide layer 505, a multiple quantum well active layer 506 where Al0.115Ga0.885As well layers (layer thickness: 74 xc3x85, two layers) and an Al0.346Ga0.654As barrier layer (layer thickness: 54 xc3x85, one layer), which are not shown, are alternately arranged, an Al0.328Ga0.672As upper guide layer 507, a p-Al0.476Ga0.524As first upper clad layer 508 and a p-GaAs etching stop layer 509 are successively laminated on an n-GaAs substrate 501. Further, a mesa-stripe-shaped p-Al0.476Ga0.524As second upper clad layer 510 is provided on the etching stop layer 509, and a visor-shaped p-GaAs cap layer 511 is formed on the second clad layer 510. Moreover, an n-Al0.685Ga0.315As first current block layer 512 and an n-GaAs second current block layer 513 are laminated on the etching stop layer 509 located on both sides of the second upper clad layer 510, making the region other than the mesa stripe serve as a current constricting portion. Moreover, a p-GaAs flattening layer 514 is provided on the second current block layer 513, and a p-GaAs contact layer 515 is laminated on the entire surface.
FIG. 5 shows a graph of the optical output-current characteristic of this semiconductor laser device. As is apparent from FIG. 5, this semiconductor laser device has a threshold current of about 35 mA and a COD (Catastrophic Optical Damage) level, i.e., an end surface destruction level of about 160 mW.
However, in the aforementioned conventional semiconductor laser device, the COD level has been able to be improved up to only 160 mW although the device has a high power. In the aforementioned conventional AlGaAs-based semiconductor laser device, end surface destruction due to COD is easily caused by the influence of active Al in a high-output driving time. Therefore, it is a basic design guideline for the obtainment of a high-output semiconductor laser device to weaken optical confinement by reducing a value of xcex93/d standardized by dividing an optical confinement coefficient (assumed to be xcex93) by a well layer thickness (assumed to be d) or reducing the well layer thickness in order to reduce the optical power density at the end surface.
However, a large gain cannot be obtained if the optical confinement is weakened to make the COD level equal to or larger than 160 mW, and therefore, a threshold current increases to disadvantageously increase the drive current. If the well layer thickness is reduced, then the overflow of carriers easily occurs to disadvantageously degrade the temperature characteristic. Accordingly, there has been a problem that only a semiconductor laser device of a large drive current has consequently been able to be provided if it has been attempted to operate the AlGaAs-based semiconductor laser device with an output of not smaller than 160 mW. Therefore, the aforementioned conventional semiconductor laser device has practically been able to stably operate with a peak optical output of about 120 mW.
Accordingly, the object of the present invention is to provide a high-output semiconductor laser device that employs a GaAs substrate or, in particular, a 780-nm band high-output semiconductor laser device for CD-R/RW, which has high reliability and long operating life in a high-output drive state and has a small drive current and an optical disk reproducing and recording apparatus that employs the semiconductor laser device.
In order to achieve the aforementioned object, the present invention provides a semiconductor laser device, which is constructed by laminating at least a first conductive type lower clad layer, a quantum well active layer comprised of well layers and barrier layers and a second conductive type upper clad layer on a GaAs substrate and has an oscillation wavelength that is greater than 760 nm and smaller than 800 nm, wherein
the well layers and the barrier layers are made of InGaAsP, InGaP or GaAsP, and
assuming that the well layer has a layer thickness d and an optical confinement coefficient in each one layer of the well layers is xcex93, then xcex93/d is not smaller than 2.2xc3x9710xe2x88x924 xc3x85xe2x88x921.
According to the above-mentioned construction, the optical confinement becomes great to allow a large gain to be obtained, because xcex93/d is not smaller than 2.2xc3x9710xe2x88x924 xc3x8531 1. Therefore, a high-output semiconductor laser device with a high COD level can be provided with satisfactory threshold characteristic and temperature characteristic maintained.
Moreover, there can be provided a high-output semiconductor laser device, which has high reliability and long operating life and in which oxidation hardly occurs, because the quantum well active layer is constructed of InGaAsP, InGaP or GaAsP and Al is not contained therein.
In the present specification, the first conductive type means the n-type or the p-type. When the first conductive type is the n-type, the second conductive type is the p-type. When the first conductive type is the p-type, the second conductive type is the n-type.
In one embodiment, the layer thickness of the well layer is not smaller than 80 xc3x85 and not greater than 200 xc3x85.
According to the above-mentioned embodiment, the well layer has the great layer thickness of not smaller than 80 xc3x85 and not greater than 200 xc3x85. Therefore, a high-output semiconductor laser device with a high COD level can be provided with satisfactory threshold characteristic and temperature characteristic maintained by reducing the overflow of carriers.
Moreover, in one embodiment, the well layer has a compressive strain.
According to the above-mentioned embodiment, the well layer has a compressive strain, and the quantum well active layer becomes a compressive strain quantum well active layer. Therefore, a semiconductor laser device, which has high reliability, long operating life and high output in the 780-nm band can be provided.
Moreover, in one embodiment, an amount of the compressive strain owned by the well layer is within 3.5%.
According to the above-mentioned embodiment, the amount of compressive strain is within 3.5%. Therefore, a high-output semiconductor laser device with high reliability and long operating life can be provided.
Moreover, in one embodiment, the barrier layer has a tensile strain.
According to the above-mentioned embodiment, the barrier layer is a tensile strain barrier layer, and the tensile strain of the barrier layer compensates for the amount of compressive strain of the well layer that has the compressive strain. Therefore, a strained quantum well active layer having a more stable crystal can be produced, and a semiconductor laser device with high reliability can be provided.
Moreover, in one embodiment, an amount of the tensile strain owned by the barrier layer is within 3.5%.
According to the above-mentioned embodiment, the amount of tensile strain is within 3.5%. With this arrangement, a high-output semiconductor laser device with high reliability and long operating life can be provided more suitably.
Moreover, in one embodiment, an upper guide layer is provided between the upper clad layer and the quantum well active layer or a lower guide layer is provided between the lower clad layer and the quantum well active layer or both of the upper guide layer and the lower guide layer are provided, the guide layers each being made of AlGaAs, and a portion that belongs to the quantum well active layer and is put in contact with the guide layer is a barrier layer.
According to the above-mentioned embodiment, the guide layer is made of AlGaAs, and the portion that belongs to the quantum well active layer and is put in contact with the guide layer is the barrier layer. The guide layer made of AlGaAs is not adjacent to the well layer where radiative recombination occurs. Therefore, the overflow of carriers can sufficiently be restrained by the energy (Ec) of the conduction band and the energy (Ev) of the valence band of the guide layer made of AlGaAs, thus reliability being secured. Therefore, a high-output semiconductor laser device with high reliability and long operating life is obtained.
Moreover, in one embodiment, an Al crystal mixture ratio of AlGaAs that constitutes each of the guide layers is greater than 0.2.
According to the above-mentioned embodiment, the Al crystal mixture ratio of AlGaAs that constitutes the guide layer is greater than 0.2. Therefore, a high-output semiconductor laser device with high reliability and long operating life can be provided more suitably.
In one embodiment, the lower clad layer is comprised of a plurality of layers made of AlGaAs of varied Al crystal mixture ratios.
According to the above-mentioned embodiment, the lower clad layer is constructed of a plurality of layers made of a plurality of sorts of AlGaAs of varied Al crystal mixture ratios, thus restraining the absorption of light by the substrate in a high-output stage by effectively shielding the laser light leaking toward the substrate. Accordingly, there can be provided a semiconductor laser device, which has higher reliability, longer operating life and a higher output.
The present invention also provides a semiconductor laser device, which is constructed by laminating at least a first conductive type lower clad layer, a quantum well active layer comprised of well layers and barrier layers and a second conductive type upper clad layer on a GaAs substrate and has an oscillation wavelength that is greater than 760 nm and smaller than 800 nm, wherein
the well layers and the barrier layers are made of InGaAsP, InGaP or GaAsP, and
the well layer has a layer thickness of not smaller than 80 xc3x85 and not greater than 200 xc3x85.
According to the above-mentioned construction, there can be provided a high-output semiconductor laser device, which has high reliability and long operating life, because the quantum well active layer is constructed of InGaAsP, InGaP or GaAsP and oxidation hardly occurs since Al is not contained.
Furthermore, the well layer has the great layer thickness of not smaller than 80 xc3x85 and not greater than 200 xc3x85. Therefore, a high-output semiconductor laser device with a high COD level can be provided with satisfactory threshold characteristic and temperature characteristic maintained by reducing the overflow of carriers.
Moreover, in one embodiment, the layer thickness of the well layer is not smaller than 150 xc3x85 and not greater than 200 xc3x85.
According to the above-mentioned embodiment, the well layer has the greater layer thickness of not smaller than 150 xc3x85 and not greater than 200 xc3x85. Therefore, a semiconductor laser device, which has a high COD level, high reliability, long operating life and a high output can be provided more suitably.
The optical disk reproducing and recording apparatus of the present invention includes the aforementioned semiconductor laser device.
In the optical disk reproducing and recording apparatus of the above-mentioned construction, the semiconductor laser device operates with a higher optical output than the conventional semiconductor laser device, and therefore, data read and write can be achieved even if the rotating speed of the optical disk is made higher than in the conventional case. Therefore, the optical disk reproducing and recording apparatus of the present invention can be operated more comfortably since the access time to the optical disk, which has been a problem particularly during write, is shortened more significantly than in the optical disk reproducing and recording apparatus that employs the conventional semiconductor laser device.
In the optical disk reproducing and recording apparatus of one embodiment, a peak optical output during recording is not smaller than 120 mW at a laser emission end surface of the semiconductor laser device.
According to the above-mentioned embodiment, the peak optical output during recording is not smaller than 120 mW or more at the laser emission end surface of the semiconductor laser device. Therefore, data read and write can be achieved even if the rotating speed of the optical disk is made much faster than in the conventional case.